what does brightness has to do with how long battery lasts? you can have max brightness and not use your phone and it'll last you whole day or have it on minimum playing games and burn through it in 4-5 hours.

if you want to find out how brightness effects battery drain you need to have two controlled tests where usage is identical the only variable is brightness. fully charge and leave it alone with screen on on lowest and highest brightness till battery is dead to find out your effect.